1. Field of the Invention
The invention generally relates to a voltage reference circuit and, in particular, the present invention relates to a voltage reference circuit with improved line regulation and having minimal operating voltage and current.
2. Background of the Invention
FIG. 1 is a conventional bandgap voltage reference circuit 10 for providing a reference voltage that is relatively constant over a sufficiently large temperature range. Bandgap reference circuit 10 includes a first bipolar transistor Q1 and a second bipolar transistor Q2 having their base terminals connected together. A current mirror formed by PMOS transistors M1 and M2 causes a first current I.sub.C1 which flows through transistor M1 to be mirrored as an identical second current I.sub.C2 which flows through transistor M2. Appropriate start-up circuitry (not shown) is provided to ensure that circuit 10 does not remain in the unstable equilibrium point where currents I.sub.C1 and I.sub.C2 are equal to zero.
In bandgap reference circuit 10, the emitter area of Q1 is made to be n times the emitter area of Q2, causing different current densities to flow in the transistors since the current mirror forces the currents of transistors Q1 and Q2 to be equal. Typically, the transistor size ratio n is in the range of 2 to 10. The unequal current densities of transistors Q1 and Q2 imply that if their currents are to be equal as required by the operation of the current mirror, then the base to emitter voltages (V.sub.BE) of Q1 and Q2 must be different. The difference in the base to emitter voltages, denoted .DELTA.V.sub.BE, is developed across a resistor R1. The voltage .DELTA.V.sub.BE is given by the equation: ##EQU1## where k is the Boltzmann's constant, T is temperature in Kelvin, and q is the electric charge. The same current that flows in resistor R1 also flows in resistor R2 and a voltage V.sub.R2 develops across resistor R2. Voltage V.sub.R2 is proportional to voltage .DELTA.V.sub.BE according to the equation: ##EQU2## where R1 and R2 represent the resistance of resistors R1 and R2.
The output voltage Vout at node 16 is the sum of the voltage V.sub.R2 and the voltage V.sub.BE of transistor Q2. It is well known that the voltage V.sub.BE of a bipolar transistor has a negative temperature coefficient while the voltage .DELTA.V.sub.BE has a positive voltage coefficient. By properly ratioing the resistance of resistors R1 and R2, a constant output voltage having approximately zero temperature coefficient can be obtained over a wide range of temperatures. For circuit 10 of FIG. 1, a nominal output voltage of approximately 1.25 volts is realized at node 16.
It is desirable to provide a bandgap voltage reference circuit where the operating voltage and operating current of the reference circuit can be kept at minimal levels, i.e., the reference circuit must consume minimal power in operation. Voltage reference circuit 10 of FIG. 1 achieves low power consumption by utilizing only two current paths from the power supply Vs (node 12) to the ground potential (node 14). By providing only two current paths, the supply current which flows in circuit 10 in operation is kept low. Voltage reference circuit 10 also allows the supply voltage Vs to be at a minimum by using PMOS transistors to implement the current mirror (i.e., transistors M1 and M2). PMOS transistor M2 requires only a minimal voltage between supply voltage Vs (node 12) and voltage Vout (node 16) for its operation as a current mirror. Furthermore, in reference voltage circuit 10, PMOS transistors M1 and M2 are long channel devices so as to maximize their output impedance.
However, conventional voltage reference circuits such as reference circuit 10 exhibit poor line regulation characteristics. Line regulation is defined as the dependence of the output voltage (Vout) on the power supply voltage (Vs). Reference voltage circuit 10 has poor line regulation characteristics due to the finite Early voltage value of transistor Q1. The Early voltage (V.sub.A) is an extrapolated voltage parameter modeling the variation of the collector current I.sub.C with respect to the collector to emitter voltage V.sub.CE in a bipolar transistor. For a description of the Early voltage of a bipolar transistor and the Early effect on the transistor output characteristics, see Gray and Meyer, "Analysis and Design of Integrated Circuits," 2nd ed., 1984, John Wiley & Sons, Inc., pages 18-19.
Referring to FIG. 1, as the supply voltage Vs (node 12) varies, voltage V.sub.CE of transistor Q1 is caused to vary accordingly. Because transistor Q1 has a finite Early voltage, the change in voltage V.sub.CE of transistor Q1 causes its collector current I.sub.C1 to also vary. Because current I.sub.C1, is mirrored to current I.sub.C2 of transistor Q2, current I.sub.C2 flowing through resistor R2 varies as a result of the variation in supply voltage Vs. Therefore, the output voltage Vout (node 16) of voltage reference circuit 10 varies with the supply voltage Vs. Even though voltage reference circuit 10 is capable of providing a constant output voltage over a wide range of temperature, the output voltage exhibits the undesirable characteristics of poor line regulation.
Prior art solutions to control the line regulation character are unsatisfactory because the solutions involve placing additional voltage or current burdens on the circuit, making it undesirable for minimal power operations.
Therefore, it is desirable to provide a voltage reference circuit with improved line regulation characteristics which can be operated with minimal operating voltage and current.